Multi-user multiple-input multiple-output (MU-MIMO) transmission is becoming a new system technique to enable high system capacity in both the upcoming IEEE 802.11ac and the LTE (long-term evolution) standards. As compared to single-user MIMO (SU-MIMO), MU-MIMO has several key advantages. First, MU-MIMO allows for a direct gain in multiple access system capacity proportional to the number of access point antennas. Second, MU-MIMO allows the higher degree spatial multiplexing gain to be obtained without the need for higher number of antennas at the mobile stations by keeping the intelligence and cost at the access point. Third, MU-MIMO appears immune to most propagation limitations plaguing SU-MIMO communications because multiuser diversity can be extracted even in a simple line of sight (LOS) propagation environment. As a result, the LOS propagation, which causes degradation in single user spatial multiplexing schemes, is no longer a problem in the multiuser setting.
FIG. 1 (Prior Art) illustrates a typical downlink MU-MIMO process adopted by IEEE 802.11ac systems. For a Time-Division Duplexing (TDD) system, it is not possible to simultaneously transmit and receive signals in the access point or router. To avoid simultaneous transmission and reception, all transmitted downlink spatial streams need to end concurrently to preventing any mobile station from transmitting a signal back during the downlink stream transmission. This is accomplished through a process called padding. As illustrated in FIG. 1, the shorter spatial streams (i.e., SS3 for STA 3 and SS2 for STA2) are appended with extra non-information bearing data to fill up the same length as that of the longest spatial stream (i.e., SS1 for STA 1). The process of padding and un-padding the downlink spatial streams at the transmitter and the receiver are stipulated in the IEEE 802.11ac specifications. For uplink transmission, however, although MU-MIMO transmission is possible, it is currently not included in the IEEE 802.11ac specifications due to difficulties in the timing synchronization. Instead, as illustrated in FIG. 1, the multiple mobile stations transmit uplink acknowledgements (i.e., block acknowledge (BA)) sequentially (i.e., separated by SIFS (short interframe space) intervals or RIFS (reduced interframe space) intervals) in response to the MU-MIMO downlink transmission in a scheduled or polled time division multiple access (TDMA) fashion.
In contrast to the SU-MIMO transmission, where the mobile station receivers are equipped with sufficient number of antennas (equal to or greater than the number of spatial streams) and the capability of the signal processing to estimate the channel and to separate the spatial streams, it is crucial in a MU-MIMO transmission for the access points or routers to bear the most of the burden in the signal processing and hardware complexity to allow for simpler mobile station implementation. To achieve this aim, the access point or router should apply transmit beamforming (precoding), computed from channel information acquired in the MU-MIMO downlink transmission to achieve an orthogonal (or near-orthogonal) transmission of multiple streams to multiple users, i.e., eliminating (or reducing) the amount of mutual interference between the transmission to multiple mobile stations. Under this condition, each mobile station only receives the spatial stream(s) intended for itself and not the interference from the spatial stream(s) intended for other mobile stations. With reduced number of spatial streams directed toward individual mobile stations, all mobile stations only need to be equipped with sufficient number of antennas for processing the spatial streams intended for itself and not worrying about eliminating the interference from other spatial streams.
The processing of multiple spatial streams at the receiver is well known to those skilled in the art. In widely deployed IEEE 802.11n systems, low cost mobile stations with SU-MIMO capability are equipped with multiple antennas and the ability to process multiple received spatial streams intended for itself, i.e., the capability for low degree of spatial processing. The commonly used receiver processing algorithms include linear processing such as zero forcing (ZF) or minimum mean square error (MMSE) and more complex nonlinear processing based on maximal likelihood receiver. For SU-MIMO, the transmit beamforming (precoding) can be optionally applied at the transmitter to mitigate the effects of noise enhancement issues in the linear processing receiver thereby enabling the linear receiver to achieve the performance of the maximal likelihood receiver.
Although the legacy 802.11n mobile stations possess the similar capability for processing the received MIMO spatial streams as the 802.11ac mobile station, they do not have the capability to process the padding and un-padding of spatial stream and to handle the scheduled or polled uplink response. This is the key obstacle for performing MU-MIMO to the legacy 802.11n devices. A solution is sought to enable downlink MU-MIMO transmission for both 802.11ac and legacy 802.11n systems to achieve enhanced system capacity.